The present invention relates to a method of manufacturing a MOS semiconductor device and, more particularly, to a method of forming a gate insulating film.
A MOS semiconductor device has been developed to obtain a high integration density, high performance, and a large number of functions. MOS transistor characteristics have many various demands. For example, a demand for a MOS transistor (Japanese Patent Laid-Open No. 62-256476) having two types of gate oxide films having different thicknesses has been made as one of the many various demands.
A conventional method of manufacturing a MOS semiconductor device having two types of gate oxide films having different thicknesses is shown in FIGS. 2A to 2E. The conventional method will be described below with reference to FIGS. 2A to 2E. As shown in FIG. 2A, an element isolation region having an element isolation insulating film 2 and an element region having a first oxide film 3 are formed on a semiconductor substrate 1 of a first conductivity type. Subsequently, as shown in FIG. 2B, the first oxide film 3 is selectively etched using a photoresist 4 by, e.g., hydrofluoric acid. As shown in FIG. 2C, the photoresist 4 is removed, and a second oxide film 5 is formed by a thermal oxidation method. At this time, the thickness of the first oxide film 3 is increased to obtain a first oxide film 3a having a large thickness. Thereafter, as shown in FIG. 2D, a gate electrode 10 made of polysilicon is formed. As shown in FIG. 2E, diffusion layers 11 prospectively serving as a source and a drain are formed, and an insulating interlayer 12 is formed. Contact holes 12a are formed, wiring electrodes 13 are formed, and a covering insulating film 14 is formed as a protection film.
According to the conventional method of manufacturing a MOS semiconductor device, when the second oxide film is formed by a thermal oxidation method, the first oxide film is thermally oxidized, and the thickness of the first oxide film is increased to cause the following problems.
1 The thickness of the first oxide film depends on the thickness of the second oxide film, and the thickness of the first oxide film cannot be independently set. That is, since the thickness of the first oxide film must have a predetermined thickness after the second oxide film is formed, the thickness of an oxide film previously formed must be adjusted in consideration of the thickness of the second oxide film. If the thickness of the second oxide film is to be changed, when the thickness of the oxide film previously formed is not changed, the thickness of the first oxide film is changed. PA1 2 Since the first oxide film is formed by two oxidation steps, variations in thicknesses of the films are larger than those of films formed by one oxidation step.